The present invention relates to a display device, and more particularly, to an active matrix type display device and a method of manufacture thereof
In a typical liquid crystal display device, on a liquid-crystal-side surface of one of a pair of transparent substrates, which are arranged to face each other in an opposed manner with a liquid crystal material disposed therebetween, gate signal lines extend in the x direction and are arranged in parallel in the y direction and drain signal lines extend in the y direction and are arranged in parallel in the x direction, and respective regions surrounded by these signal lines define pixel regions.
On each pixel region, there is at least a thin film transistor, which is operated in response to scanning signals from a one-side gate signal line and a pixel electrode to which video signals are supplied from a one-side drain signal line through the thin film transistor. The pixel electrode generates an electric field between the pixel electrode and a counter electrode thus controlling the light transmittivity of the liquid crystal material. Further, as the thin film transistor, a transistor has been employed which uses a semiconductor layer which is referred to as a so-called a low-temperature polysilicon (p-Si) layer. Such a thin film transistor can be formed by a low temperature process at a temperature of not more than approximately 450° C.
There is a known liquid crystal display device in which a scanning driving circuit, which supplies scanning signals to the gate signal lines, and a video driving circuit, which supplies video signals to the drain signal lines, are formed on one of the above-mentioned substrates. Each driving circuit is comprised of a large number of complementary MIS transistors, because these MIS transistors can be formed along with the formation of the above-mentioned thin film transistors. As the constitution of such thin film transistors, the constitution which is disclosed in Japanese Laid-open Patent publication 163366/1999 has been known, for example.
With respect to a thin film transistor having such a constitution, so-called LDD (Lightly Doped Drain) regions are formed respectively between a channel region thereof and drain and source regions, which are formed at both sides of the channel region, and the widths of respective LDD regions are made uniform so as to make the magnitude of ON currents uniform.
These LDD regions are regions which are doped with an impurity having a concentration lower than the concentration of the impurity doped into the drain and source regions. The LDD regions are formed to alleviate the concentration of an electric field at these portions.
However, with respect to such a thin film transistor, no consideration has been given to the film thickness of an insulation film (functioning as a gate insulation film) which covers a channel region, an LDD region and the drain and source regions thereof. Accordingly, it has been pointed out that the areas of the tapered surfaces of contact holes cannot be reduced, so that the numerical aperture cannot be enhanced, or there arises a defect with respect to the coating ability of an interlayer insulation film due to the formation of a stepped portion in the periphery of a gate electrode of the thin film transistor.
The present invention has been made in view of such circumstances as described above, and it is an object of the present invention to provide a display device which can enhance the numerical aperture and can resolve defects which occur in a periphery of a gate electrode of a thin film transistor.
Further, it is another object of the present invention to provide a method of fabricating a display device which can reduce the voltage necessary for ion implantation of an impurity at the time of forming the thin film transistors.